Run Silent, Run Clean: Microsubmarines Scoop Up Oil

Rachel Nuwer|Fri May 4 2012

Oil contamination pollutes groundwater, damages sea life and kills birds. Though a bane to industry and environment, mopping up the estimated 1.3 million gallons of petroleum spilled into U.S. waters each year (that’s in a good year) is no easy task. Cleaning methods are expensive, ineffective and even harmful to the environment. But nanoengineeers think they’ve hit upon a new technology that could help out: microsubmarines designed to capture oil, one droplet at a time.

“They’re self-propelling, so you send them out and on their way they’ll pick up the oil droplets—eventually they’ll collect everything,” said Joseph Wang, a nanoengineer at the University of California-San Diego.

Wang and his team have been working on these so-called tubular microengines for a decade. They were one of the first labs to pioneer the new technology, which started as an idea for delivering targeted drug therapy or even removing cancer cells by traversing a person’s bloodstream like a biological highway. The current venture, however, is the first Wang knows of that applies nanomachines towards environmental remediation.

The subs have a little bling, too. Their surface is plated in gold for snagging oil droplets.

The tiny machines are about one tenth the width of a human hair, or 8 micrometers long. The cone-shaped tubes have a platinum inner surface that oxidizes hydrogen peroxide fuel to create “bubble-induced propulsion.” Those bubbles are no joke, either—the little tubes zip around at ultrafast speeds that take them distances up to 1,400 times their body length in a second. “The bubble propulsion is amazing!” Wang said.

The subs have a little bling, too. Their surface is plated in gold, which gives them a rough texture perfect for snagging oil droplets. But the gold in itself is not sufficient to do the trick; normally, the microsubmarines would simply pass through oil without collecting it, as they did in control trials. To make them “superhydrophobic,” or extremely water repellant but appealing to oil, Wang coated them in a special alkanethiol polymer chain. The end of the polymer chain is very attractive to oil droplets, which get stuck to it like rockachaws on a sock as the little robots pirouette around the spill. By tweaking the type of polymer chain used, the researchers can optimize the robots for targeting various types and densities of oily contaminants.

Wang and his team performed experiments with their new creations, placing them in dishes containing olive oil and motor oil. The mini subs swam around like otters, gathering up to 80 oil droplets that dragged behind them in lengthy chains. Thanks to Stokes’ law of drag force, the larger the cargo size, the slower the robots were able to swim. Still, they were able to handle a towing force up to 10-fold their volume. When it was time to call the microengines back to unload the oil, Wang exploited a magnetic layer situated between the gold and platinum. “The magnetic guidance allows us to navigate like a steering wheel,” he said. After dumping the oil, the machines can be used over and over again.

The study is just a proof of concept. Some tweaking remains.

The study, which was published in the American Chemical Society’s journal Nano, is just a proof of concept. Unfortunately, there will need to be some tweaking before we’re ready to send an army of microscopic robots out into the Gulf. Technical problems still abound, like how many millions of microscopic machines would be needed to tackle an entire spill. Wang also suggests it may be possible to scale them up in size in order to be more efficient.

Another issue is environmental impediments, like waves or strong currents. Wang’s team showed in trials that the microsubmarines can swim against a stream’s flow, but whether they could weather a tropical storm is a different question. The team also hopes to replace the robots’ dependence on hydrogen peroxide, eventually finding a fuel-free solution, like wave energy or a magnetic tail that rotates like that of an energetic spermatozoon.

Despite the challenges ahead, Wang is optimistic. Eventually this technology could be used not only to clean up oil spills, but to remove contaminants within the human body, or even undertake a “seek and destroy” mission targeting toxic bacteria, he said. Years down the line, he sees opportunity for private sector investment and commercialization of the technologies.

“We’ve developed all of these capabilities, and more recently we’ve been finding specific applications for them,” he said. “It’s an exciting time with much more to come, so stay tuned.”

Top image: A slightly larger version of the microsubs. Courtesy Flickr user MATEUS_27:24&25

Rachel Nuwer is a science journalist who writes for venues including the New York Times, ScienceNOW and Audubon Magazine. She lives in Brooklyn, NY. She tweets @RachelNuwer.